diff --git a/lib/RelaxNG/openmath2.rnc b/lib/RelaxNG/openmath2.rnc
index 62a57d1..78064f6 100644
--- a/lib/RelaxNG/openmath2.rnc
+++ b/lib/RelaxNG/openmath2.rnc
@@ -1,4 +1,5 @@
 # RELAX NG Schema for OpenMath 2
+# Revision 2: Corrected regex for OMI to match the documented standard and allow hex
 
 
 default namespace om = "http://www.openmath.org/OpenMath"
@@ -41,7 +42,7 @@ OMV = element OMV { common.attributes,
 
 # integer
 OMI = element OMI { common.attributes,
-                    xsd:string {pattern = "\s*(-\s?)?[0-9]+(\s[0-9]+)*\s*"}}
+                    xsd:string {pattern = "\s*-?((\s*[0-9])+|x(\s*[0-9A-F])+)\s*"}}
 # byte array
 OMB = element OMB { common.attributes, xsd:base64Binary }